1. Field of the Invention
The present invention relates generally to cellular telephony and, more particularly, to controlling power consumption in cellular telephony.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Mobile or cellular telephones are widely used in today's society. The first mobile phone services appeared in the United States in the mid-1940s. Back then, the cost of each mobile phone was quite high, and as a result, the market for mobile phones remained small. To serve the limited mobile phone market, mobile services within a metropolitan area used a single tower or “base station.” Each mobile phone communicated with the base station using a single dedicated radio frequency (RF).
As technology improved, mobile phones became smaller and less expensive, allowing them to become more plentiful. Since each mobile phone communicated with the base station via a single frequency, the number of frequencies required to support the growing number of phones also increased. However, the number of frequencies available in the RF spectrum is limited, both physically and by regulatory agencies such as the Federal Communications Commission. To address these issues, the Advanced Mobile Phone Service (AMPS) was introduced in 1970, and AMPS became America's mobile phone standard. The AMPS system solved the problem with the limited number of available frequencies by scattering multiple low-power base stations throughout a metropolitan area, where mobile phone calls are “handed off” as the mobile phone travels around the metropolitan area. Thus, each base station defines a “cell,” and a customer's mobile or “cellular” phone communicates with different base stations as the customer's call is handed off from one cell to another.
In general, handoffs can be classified as either “hard” or “soft.” In a hard handoff, the connection between the mobile phone and its current cell is broken prior to establishing a connection with a new cell. Conversely, a soft handoff includes making a connection to a new cell before breaking the connection with the current cell, thus allowing the mobile phone to be connected to at least two or more base stations concurrently.
Many different types of cellular access systems exist, where each cellular access system may implement one or more of the different types of handoffs. The fundamental difference between each of the available cellular access systems is their use of two common resources, frequency and time. Frequency division multiple access (FDMA) divides the frequency and allocates each mobile phone a portion of the frequency spectrum for all of the time available. Time division multiple access (TDMA) divides the time, so that each mobile phone receives a portion of the available frequency spectrum for a portion of the available time. Code division multiple access (CDMA) allows each mobile phone to transmit over the entire frequency spectrum for the entire time. Using unique spreading codes to spread the low frequency or “baseband” data (e.g., voice information) before transforming the data to RF for subsequent data transmission. Since each code is unique, multiple users may share the entire frequency bandwidth available all of the time.
As can be appreciated, FDMA allows the entire frequency spectrum to be allocated to a single mobile phone. As a result, any given communication channel (frequency or time) cannot be reused by neighboring cells. Thus, in FDMA, the mobile phone will be commanded to terminate communication with one cell prior to establishing connection with another cell—i.e., perform a hard handoff. Unlike FDMA, CDMA has a “soft” capacity because frequency and time can be shared. That is, there is no hard limit to how many users allow on the system. Accordingly, in CDMA, the mobile phone can communicate with multiple base stations simultaneously to perform a soft handoff. In this respect, CDMA typically has the advantage of having more system capacity than the other multiple access schemes.
The practical limit on the number of mobile phones that a CDMA system can support is based on the amount of interference or noise present on the system. Specifically, as the noise increases, the system capacity decreases. Since all mobile phones transmit on the same frequency, decoding a single mobile phone signal includes distinguishing that particular signal from all received mobile phone signals. In other words, non-desired mobile phone signals are simply noise with respect to the desired mobile phone signal. Therefore, distinguishing between desired and non-desired signals becomes increasingly difficult as more mobile phones are added to the system because the system noise increases. The amount of interference caused by phone transmissions may be reduced by keeping the transmission power of the phones low. Conversely, the transmission power of each phone should be kept high enough so that the base station can retrieve the phone's signal without error. That is, the phone's transmission power should be controlled so as to achieve a desired signal-to-noise-ratio (SNR).
Power control techniques are used in CDMA, and other spread spectrum systems, for other reasons. Since phones in the CDMA system use the same frequencies, CDMA systems have a “near-far” problem. That is, phones that are near the base station may overpower phones that are farther from the base station if their transmission power of the phones is not controlled properly. For example, assume that two phones transmit the same amount of power, but one phone is substantially closer to the base station than the other. In a CDMA system, or any other system where these two phones use the same frequencies, the phone that is nearer to the base station could overpower the phone that is farther from the base station. Accordingly, a method of controlling cell phone transmission power is desirable.